The present invention relates generally to microwave circuits and, more particularly, to an assembly for covering a housing which encloses a microwave circuit, wherein the cover assembly includes externally-accessible probes for providing tuning of the microwave circuit while the cover assembly is in place on the housing.
A typical microwave hybrid integrated circuit (MHIC) module may comprise a plurality of ceramic substrates, the substrates including resistive, capacitive and active components soldered to a surface and further including gold circuit traces deposited on a polished surface for interconnecting the components. The component-mounted substrates are typically mounted side by side in an aluminum housing, interconnected by gold jumper wires and enclosed within the housing via an aluminum cover. At the microwave frequencies of interest, the geometry of the gold circuit paths on the substrates, and their locations with respect to each other and to the surrounding housing (particularly including the cover), creates inductive and capacitive effects.
The total assembly, including the component-mounted substrates within the housing, is tuned for gain and "noise figure" over the frequency band of interest. This tuning may typically be accomplished by connecting jumper wires from the main circuit traces to small tuning patches adjacent a trace path or at the end of a branch. Although the tuning patches are quite small, illustratively 0.02 in. (0.5 mm) by 0.06 in. (1.5 mm), they have a significant influence on the performance of the circuit when they are connected, by slightly increasing the area or length of the associated trace.
Unfortunately, the subsequent installation of the cover on the aluminum housing after the above-described tuning procedure has a far greater impact on circuit performance than the interconnection of the internal tuning patches and jumpers. Clearly, the cover must be in place before the assembly can be finally tested, but the presence of the installed cover would ordinarily preclude further adjustment, as it covers the circuits.
It has been the practice in the past to make educated guesses regarding the soldering of tuning patches to the circuit traces during a series of iterations involving the affixing of the cover, testing for the above-described gain and noise parameters, and removal of the cover. This time-consuming procedure may be further exacerbated by a large number of cover screws, all of which must be completely in place before the electrical testing may be performed with accuracy. It is not unusual to find that an average of eight such tuning iterations are required for each MHIC module before it meets its electrical specification.
Clearly, this form of iterative tuning procedure creates a bottleneck in any circuit production facility. What is required is a real-time procedure for tuning the circuits via jumpers to the tuning patches while in an environment which accurately simulates the enclosed housing.